He made a similar announcement last October, but changed his mind in the face of an outpouring of support from his fans on social media. This time, however, “it’s the real deal,” he wrote in the Sept. 11 letter, which was first reported by the trade publication Current.

By email, Mr. Moyers said, “A few months ago I began to hear what Shakespeare described as that ‘inaudible and noiseless foot of time’ that only you can identify as the stranger suddenly trotting beside you.”

He added that he still relished the work and believed he still did it well, but “while I have sometimes been foolish enough to think I could keep doing it forever, I know otherwise.”

He also said that he would miss viewers, his funders and his colleagues. “But I have been at this 43 years,” he said, “I am 80 years old, and the simple truth is: It’s time to go.”

His website, billmoyers.com, which contains an archive of much of his public television programming since 1971, will remain up “for the indefinite future,” Mr. Moyers wrote in the letter to station managers, and will continue to add new content.